Inside the Sleep and Mood Research Clinic at the University of California, Berkeley, researchers studied how the number of hours a person sleeps may affect their weight. The findings, published in the journal Sleep, reveal how teenagers are especially susceptible to gaining weight when they regularly go to bed late.

For the study, researchers analyzed 3,300 teens’ and adults’ sleep schedules over five years and calculated that for every extra hour that teens stayed awake, they would gain about 2.1 points on their body mass index (BMI). BMI is a scale that’s used to measure a person’s body fat based on their height, weight, and gender, and ranges from underweight (less than 18.5) to morbidly obese (30 or greater). The researchers also found that teens were still susceptible to weight gain if they stayed up late and slept in later the next day. These findings held regardless of the time the teens spent exercising, or on their computers, cell phones, tablets, or other electronic devices. If they stayed up late, they were more likely to gain weight.

A person’s circadian rhythm, also known as their natural sleep cycle, regulates physiological and metabolic functions, and typically shifts to a later sleep cycle when puberty hits. For this reason, developing healthy sleep habits as a teen lays the foundation for a healthier adulthood — with regard to both a healthy sleep pattern and other aspects of wellbeing.

According to the National Sleep Foundation’s 2013 International Bedroom Poll, the amount of sleep the average person logs each night has steadily decreased over the past century. The average American sleeps only six-and-a-half hours a night during a five-day workweek. But while Americans have been skipping their sleep, their waistlines have been getting wider. In the last 30 years alone, obesity rates have quadrupled in adolescents, adding to the 34.9 percent of the country’s adult obesity population.

"Obesity is obviously growing among adolescents and adults, and there's also an epidemic of lack of sleep and later bedtime preference in teens," the study’s lead author Lauren Asarnow, a doctoral candidate at the University of California, Berkeley, told CBS News. "There's been some literature looking at the relationship [between] late bedtimes and weight gain cross-sectionally, but no one's ever looked at what happens long-term."

Scientists are just starting to grasp the extent to which human health depends on the length and quality of rest the body gets. Sleep typically takes up about a third of every person’s day, giving merit to why research on it has been gaining momentum — this began in 1993 when the National Institutes of Health opened up a branch specifically designed to study how sleep affects health on a short and long-term basis.

Teens are also more likely to engage in late night snacking. When the brain is sleep deprived, the prefrontal cortex, which controls impulses and decision making, isn’t working at full capacity, and therefore makes a person far more likely to give into cravings. “These results highlight adolescent bedtimes, not just total sleep time, as a potential target for weight management during the transition to adulthood,” Asarnow said in a press release, adding that teens who go to bed early “set their weight on a healthier course as they emerge into adulthood.”

Source: Asarnow L, et al. Sleep. 2015.